A challenging aspect of designing advanced cellular systems is the development of a flexible and efficient physical layer modulation scheme. Some of the desirable attributes of a physical layer include: moderate peak-to-average power ratio, to reduce the cost, size, and power consumption of subscriber-unit power amplifiers; support for a wide range of data rates, and the ability to vary data rates based on received signal quality; support for adaptive modulation; support for frequency diversity of wideband channels (e.g., 20 MHz), even when transmitting at “narrowband” data rates; and the ability to maintain orthogonality between transmissions by different users in a cell, in order to minimize intra-cell interference and maximize capacity.
Interleaved frequency division multiple access (IFDMA) is a multicarrier modulation method that could potentially satisfy many of the above-listed criteria for a physical layer. IFDMA takes some of the desirable characteristics of both orthogonal frequency division multiplex (OFDM) and single carrier modulation and combines them into an improved type of modulation. With IFDMA, the baseband signal begins as a single-carrier quadrature phase shift keying (QPSK) or quadrature amplitude modulation (QAM) symbol stream. Since IFDMA starts as a single carrier scheme (prior to blocking and repetition), it has the same peak-to-average ratio as single carrier modulation, and provides frequency diversity. However, in the IFDMA scheme, every user must transmit with the same number of subcarriers. This prevents the use of different data rates by different users and limits the flexibility of IFDMA in an advanced communication system.
Therefore, there is a need for an improved modulation scheme that provides a high degree of data rate flexibility, while preserving orthogonality and a low peak-to-average ratio.